added on_delete in models an migrations for django 2.0

compatibility.

src/content/models.py

@@ -3,9 +3,9 @@ from ckeditor_uploader.fields import RichTextUploadingField
 from django.conf import settings
 from django.core.cache import cache
 from django.core.exceptions import ValidationError
-from django.urls import reverse
 from django.db import models
 from django.template.defaultfilters import slugify
+from django.urls import reverse
 from django.utils import timezone
 from django.utils.safestring import mark_safe
 from django.utils.translation import get_language, ugettext as _
@@ -69,11 +69,14 @@ class Article(models.Model):
     headline_en = models.CharField('Headline', max_length=255, blank=True)
     content_de = RichTextUploadingField(_('Content'))
     content_en = RichTextUploadingField('Content', blank=True)
-    category = models.ForeignKey('Category', verbose_name=_('Category'))
+    category = models.ForeignKey('Category',
+                                 on_delete=models.PROTECT,
+                                 verbose_name=_('Category'))
     image = models.ImageField(_('Image'), upload_to='news/',
                               blank=True, null=True)
     slug = models.SlugField(_('Slug'), unique_for_month='date_created')
     author = models.ForeignKey(settings.AUTH_USER_MODEL,
+                               on_delete=models.PROTECT,
                                verbose_name=_('Author'))
     status = models.SmallIntegerField(_('Status'), choices=STATUS_CHOICES,
                                       default=STATUS_PUBLISHED)
@@ -275,12 +278,14 @@ class Page(models.Model):
     @property
     def description(self):
         """Return the localized description, fallback to german if necessary."""
-        return getattr(self, "description_%s" % get_language()) or self.description_de
+        return getattr(self,
+                       "description_%s" % get_language()) or self.description_de
 
     @property
     def menu_name(self):
         """Return the localized menu name, fallback to german if necessary."""
-        return getattr(self, "menu_name_%s" % get_language()) or self.menu_name_de
+        return getattr(self,
+                       "menu_name_%s" % get_language()) or self.menu_name_de
 
     @property
     def pdf_file(self):